The role of breast milk leukocytes in conveying passive protection and possibly altering active immunity in the neonate will be studied especially as it affects local enteric immunity. The production and distribution of breast milk leukocytes will be studied by radioautography and the hypothesis that in the newborn these cells may transiently reside either in the gut or the systemic circulation will be tested. The effect of the leukocytes on neonatal gut bacterial colonization will be investigated and the possible mechanisms by which these cells are protective explored: effect on colonization, control of invasion, or breakdown of toxic products. Experimental models to explore the gut barrier function in the newborn rat and mouse will be developed with bacteria and viruses known to invade via the enteric route and the models studied to discriminate the role of cells and humoral factors in the passive protection conveyed by milk. The activity of breast milk macrophages and the enhancement of this activity by antibodies and lymphocytes against bacteria and viruses will be investigated in in vitro systems. The hypothesis that milk cells may transmit specific systemic cellular immunity from mother to neonate, will be tested.